The invention concerns an arrangement for automatically varying the excitation of a sweep deflection circuit (illustratively a horizontal deflection circuit for a television receiver), and more particularly to arrangements of this type employing a thyristor or other controllable switch which is associated with the deflection circuit and which is responsive to a variable-phase trigger control pulse applied to a control electrode of the switch during each cycle of the sweep waveform for varying the excitation of the deflection circuit in proportion to load-induced changes in the phase of the control pulse.
Sweep deflection circuits of known type employ a line transformer whose output voltage exhibits a repetitive waveform including a forward sweep portion and a flyback portion. The voltage generated by the transformer is fed to suitable deflection coils in a cathode ray tube or the like. Ordinarily, changes in the excitation or load conditions of the cathode ray tube lead to corresponding changes in the output voltage of the transformer.
In order to compensate for such changes in load, circuitry has been developed for varying the excitation of the deflection circuit, and thereby the voltage of the transformer, in response to changes in the operating condition of the cathode ray tube fed by the transformer. Typically, such arrangements couple, to the excitation inputs of the deflection circuit, the transconductive path of a thyristor or other thyratron-like controllable switch. The control electrode of such switch is generally coupled to a source of phase-variable trigger pulses, which in turn are derived at least in part from the flyback portion of the voltage appearing on the deflection transformer. Such trigger pulse is varied in phase in proportion to changes in the operating load conditions on the transformer from a nominal value, thereby varying the conductive interval of the switch and thus its energy interchange with the deflection circuit to ideally compensate for such load changes.
One disadvantage of known compensation circuits of this type employing controllable switches is that, in general, they require the derivation of two separate control signals (at least one of which is responsive to the flyback portion of the sweep voltage), and the subsequent comparison of such independent signals to determine the phase of the switch trigger signal. Such necessity of initially generating the two separate signals to be compared (either from separate portions of the deflection circuit or from separate portions of the sweep waveform) makes the resultant compensation circuit complicated and expensive.